In the semiconductor manufacturing industry, it is necessary to achieve precise control of the quantity, temperature and pressure of one or more reactant materials which are delivered in a gaseous state to a reaction chamber. Mass flow controllers are widely used in the semiconductor manufacturing industry to control the delivery of process reactants. In FIG. 1 there is shown an example of a typical mass flow rate controller (MFC). The MFC generally includes a mass flow rate sensor (which includes a sensor tube and bypass tube, as described below) for measuring the rate of flow of gas through the MFC, a valve for controlling the flow of gas through the MFC and a simple control circuit or a computer mounted on a P.C. board and connected to the mass flow rate sensor and the valve. The computer is programmed with a desired flow rate through a connector, for example, which the computer compares to an actual flow rate as measured by the mass flow rate sensor. If the actual flow rate does not equal the desired flow rate, the computer is further programmed to open or close the valve until the actual flow rate equals the desired flow rate.
Thermal mass flow sensors operate on the principle of conservation of thermal energy, where power applied to a flowing gas equals the mass flow rate of the gas multiplied by the specific heat of the gas, the density of the gas and the temperature change of the gas. The mass flow rate can therefore be determined if the properties of the gas, the temperature changes of the gas, and the rate of power applied to the gas are known.
One class of thermal mass flow rate sensors employs a sensor tube as the primary sensing mechanism, as shown in the exemplary prior art mass flow rate sensor 10 of FIGS. 1 and 2. In such a device, a sensor tube 12 diverts a portion 14 of the main flow 16 passing through a primary conduit 18 of the MFC, while the remainder of the flaw passes through a bypass tube 18a that includes a laminar flow element 22. It is important to note that this figure is not necessarily to scale. Typically the sensor tube 12 is significantly smaller than the primary conduit 18, but is shown somewhat large in FIG. 2 for clarity, Generally one or more heating elements 20 attach tote sensor tube 12 to allow a heat transfer from the heating elements 20, through the tube 12 and to the fluid. The heating elements 20 also serve as resistance tenperature sensors that track the local temperature of the wall of the sensor tube 12.
The increase in gas temperature between the two heating elements 20 is a function of the mass flow rate of the gas through the sensor tube 12, the specific heat of the gas, the density of the gas, and the power delivered to the heater elements 20. A circuit converts the difference in resistance (or temperature) of the two elements 20 into a voltage output (power) which is calibrated to known flow rates. Normally, the change in resistance is converted to voltage by a Wheatstone bridge, which is connected to the processor. The processor compares the voltage level to stored reference gas calibration data to determine the flow rate. The stored reference gas calibration data, or table, includes voltages produced by the sensor for a range of known flow rates of the reference gas.
Since the calibration data changes for gases other than the reference gas, a characterization of the calibration data is required for each type of gas being measured in the sensor tube 12, in order for the resulting measurement to be accurate. This characterization is also referred to as multi-gas correction functions. The multi-gas correction function is the ratio of flows, in the sensor tube 12 only, of the new gas over the reference gas (Qnew/Qref). This ratio changes with sensor voltage. The calibration table of the reference gas is simply a list of sensor voltages and measured total flows at those voltages. To obtain the calibration table in the new gas, the flow of the reference gas is multiplied by the multi-gas correction function at each voltage in the reference gas calibration table. The multi-gas correction function is meant to make the sensor tube 12 independent of the type of gas being measured.
The multi-gas correction function assumes that a bypass ratio is the same in both the reference gas and the gas being measured. The bypass ratio η (also referred to as split ratio) of the sensor 10 is defined as the total flow through the bypass tube 18a and the sensor tube 12, Qtotal divided by flow through just the sensor tube 12, Qsensor.
                              BypassRatio          ≡          η                =                                            Q              Total                                      Q              sensor                                =                                                    Q                sensor                            +                              Q                bypass                                                    Q              sensor                                                          (        1        )            
In a multi-gas application, η must be equal for all gases. Any change in η from that of the reference gas is defined as the multi-gas bypass ratio error εbp for that gas.
                              MulitgasBypassRatioError          ≡                      ɛ            bp                          =                  (                                    η              -                              η                ref                                                    η              ref                                )                                    (        2        )            
εbp translates directly into a calibration error for the new gas. The bypass tube 18a is normally designed to minimize this error.
The multi-gas bypass ratio error εbp occurs because the bypass ratio η changes for different gases because of pressure losses, such as entrance effects, caused by non-ideal geometric conditions of the primary conduit, the bypass tube and the sensor tube. These pressure losses are often referred to as “Reynolds Losses” because the losses are a function of the Reynolds number of the gas being measured. The Reynolds Losses can be a major source of error in measuring the gas flow. The Reynolds losses are normally minimized or eliminated so that the bypass ratio η remains constant for different gases by properly designing the bypass tube 18a and the sensor tube 12. Properly designing the bypass tube 18a, however, often results in a complex, relatively large and expensive sensor 10, especially at high flow ranges.
It is an object of the present disclosure to provide a new and improved thermal mass flow rate sensor which can be used with different gases. Preferably, the new and improved thermal mass flow rate sensor will be substantially independent of gas properties (i.e., characterization of the bypass ratio will not be required for each type of gas being measured in the mass flow rate sensor). In addition, the new and improved thermal mass flow rate sensor also will preferably be relatively simple in design, inexpensive to manufacture, and compact in size.